Hydromorphology

Hydromorphology and Good Status of Danube’s Waters

Hydromorphological conditions play an important role in the functioning of aquatic ecosystems and are therefore important elements with regard to water status. Undisturbed hydrological regime, river continuity and morphological conditions are a prerequisite for the formation of type-specific habitats for different species. Within the hydrological regime it is important to preserve quantity and dynamics of water flow and connection to groundwater bodies. Related to river continuity it is important to enable migration for aquatic organisms and transport of sediments and within morphological conditions to preserve river depth and width variation, structure and substrate of riverbed as well as structure of the riparian zone and connection between channel and floodplains/wetlands. Undisturbed hydromorphological conditions are not important only in relation to habitats, but also for reduction of nutrients, adaptation to climate change and water scarcity as well as for droughts prevention.

Impact of Hydromorphological Alterations

Over centuries, human activities and constructions have led to fundamental changes in the physical structure and appearance of our rivers, lakes, and coastal waters. Along the course of the Danube and its tributaries, natural habitats have been substantially decreased which is reflected in deteriorated water status and significantly reduced biodiversity. Migration routes for fish species have been blocked by diverse barriers.

Hydromorphological alterations in the Danube River Basin are mainly caused by flood protection measures, hydropower, navigation, agriculture and water supply. In some cases, development schemes that are causing hydromorphological alterations serve to multiple purposes. The following three key hydromorphological alterations of basin-wide importance have been identified, considering sequence of hydromorphological quality elements in the WFD:

• Hydrological alterations (impoundments, water abstractions, hydropeaking),
• Interruptions of longitudinal river continuity and sediment balance alterations,
• River morphological alterations and disconnection of wetlands/floodplains.

Besides already existing pressures, also new hydromorphological pressures will arise in the future due to implementation of new infrastructure projects, mainly related to flood protection, navigation and hydropower production. 

More than half of water bodies in the DRB are still under (at least one) significant hydromorphological pressure. There are 422 impoundments, 69 water abstractions and 42 cases of hydropeaking that are causing significant pressure in the DRB. Additionally, 624 river continuity interruptions are not passable for fish migration, while more than half of water bodies have altered morphological conditions.

Our Vision addressing Hydromorphological Alterations

Danube waters managed in such a way as to eliminate the negative impacts on hydromorphology and further on to the aquatic ecosystem and its natural development and distribution.

Progress from 2009 to 2021 in addressing Hydromorphological Alterations

Numerous hydromorphological measures have been implemented in the period between 2009 and 2021. The main aim of those measures was the mitigation of hydrological alterations like impoundments, water abstractions and hydropeaking, improvement of river continuity (building of fish passes), reconnection of wetlands/floodplains and improvement of morphological conditions (river restoration projects).

• 66 implemented measures were related to the improvement of hydrological alterations, mainly to impoundments and water abstractions. As of the year 2021, additional 5 measures addressing hydrological alterations are in the construction phase.
• 127 fish migration aids were completed; as for 8 fish migration aids the construction is on-going as of the end of 2021. Furthermore, numerous fish migration aids and river restoration projects are currently in the planning phase.
• 58 river restoration projects have been implemented, while additional 21 river restoration projects are in the construction phase as of the year 2021.
• There were also 61,745 ha of wetlands/floodplains partly or totally reconnected; for additional 4,526 ha the construction of reconnection is still ongoing as of the end of 2021.

While within the DRBMP only measures on rivers with a catchment area larger than 4,000 km2 are presented, it is important to emphasize that Danube countries are implementing hydromorphological measures also on other (smaller) rivers, where diverse hydromorphological pressures were assessed.

Projects and additional Activities addressing Hydromorphological Alterations

Besides implementation of listed technical measures, there were also many other research projects implemented until 2021, including the Danube Floodplain Project, the Danube Sediment Project, the MEASURES Project and others, that are bringing new developments also related to hydromorphology and serve as a basis for preparation of further research and also technical measures.

In the period between 2015 and 2021 various ICPDR activities were realized, supporting further developments in the field of hydromorphology and better implementation of hydromorphological measures in the Danube River Basin. In collaboration with Danube countries there were prepared

• analysis and recommendations on significant criteria for hydromorphological pressure assessment in the DRB,
• analysis of relationship between hydromorphological alterations and response of biological quality elements in rivers in DRB,
• catalogue of mitigation/restoration measures,
• recommendations on implementation of hydromorphological measures focusing on the financial sources availability,
• a discussion paper on coordinating the implementation of the WFD and FD,
• catalogue of lighthouse projects related to hydromorphology,
• ecological prioritisation of barriers in the DRB and others.

All these documents serve as a good basis for further improvements in the field of hydromorphology and implementation of hydromorphological measures, which are necessary for achievement of environmental objectives in the DRB.

Planned Measures addressing Hydromorphological Alterations towards 2027

Danube countries agreed on the following recommendations for management objectives and actions to be implemented by 2027 as steps towards the vision addressing hydromorphological alterations:

• Restoration/mitigation of hydrological regime and habitats to ensure improvement of water status/water potential and aquatic ecosystems.
• Construction of fish migration aids and other measures at existing migration barriers as well as removing (e.g. obsolete) barriers to achieve/improve river continuity in the Danube River and in respective tributaries to support free-flowing river sections and ensure self-sustaining sturgeon populations and specified other migratory fish populations.
• Restoration of habitats and ecological corridors for migratory fish species, in particular sturgeons.
• Further investigations of sediment balance alteration and measures for its improvement.
• Restoration/mitigation of river morphological conditions/alterations and habitats to ensure improvement of aquatic ecosystems and water status/water potential.
• Implementation of further measures for the protection and restoration of existing and the restoration of former (potential) wetlands/floodplains with reconnection potential with aim to achieve environmental goals (related to water status and biodiversity).
• Further prevention of deterioration of water status/potential from new infrastructure projects.
• Closing of knowledge gaps in the field of assessment of hydromorphological alterations, improving of monitoring programme, and exchanging of good practice examples between Danube countries.

In terms of hydromorphological pressures, further restoration measures are planned for 222 water bodies in respect of river morphology until 2027.

• In 204 cases, measures are planned to reduce the impacts of impoundments like for instance by improving river morphology in the head sections of reservoirs.
• Measures towards establishing ecological flows are envisaged for 46 water abstractions and measures to reduce the impacts of hydropeaking are planned in 32 cases.
• In total 424 restoration measures on river continuity for fish migration are planned to be implemented by 2027.
• Further 23,399 ha of wetlands and floodplains will be reconnected by 2027. In total, 144,659 ha of wetlands and floodplains have been identified for a reconnection potential in the DRB.

Hydromorphological Alterations and Climate Change

Climate change is expected to modify precipitation and snow (ice) storage, increase of evaporation and decline in groundwater storage and recharge.  These modifications together with increase in extreme weather events will influence also hydromorphological conditions and consequently water status and achievement of environmental objectives.

Negative effects of climate change will be more evident on hydromorphological altered rivers. Free-flowing rivers in protected watersheds are expected to be the most resistant and resilient to climate change. In these watersheds temperature and flow changes are buffered compared to clear-cut or urbanised watersheds. Water ecosystems with (near) natural hydromorphological conditions have also a higher purification capacity and are therefore more resilient to pollution. Thus, it is crucial to improve hydromorphological conditions and purification capacity to be prepared for new negative effects caused by climate change. Furthermore, rivers with preserved hydromorphological conditions are also more resilient to floods and droughts. They have positive impacts on status of groundwater bodies and are also causing local cooling effect.

Hydromorphological measures, such as adaptation of land use in river basin, restoration of former (potential) floodplains  and conservation of actual floodplains, river revitalisation, creation of buffer strips (riparian zones), revitalisation of drainage systems and implementing of ecological flow positively contribute to minimize the effects of climate change.

For some hydromorphological measures, there is also a close link to measures foreseen in FRMPs (e.g. restoration of former (potential) floodplains, conservation and restoration of actual floodplains) and link to drought management, while in general all hydromorphological measures support goals of EU Biodiversity Strategy for 2030. Implementation of concept “Giving more space to rivers“ is recognized as one of the most important recommendations for achieving common synergies and benefits between WFD and FD objectives. To reduce flood risk, high priority has to be put on a sound planning process followed by non-structural (spatial planning) measures. Emphasis has to be put on green measures (natural based solutions – NBS and natural water retention measures – NWRMs). However, for densely populated areas or areas with high land use pressure, and no availability of retention areas at hand, structural measures have to be applied by means of flood risk reduction.

For successful implementation of listed hydromorphological measures, there is need for good cooperation among multiple partners from different sectors. Especially in the light of climate change, it is also important to raise awareness for the necessity to increase the funding available for hydromorphological issues to a level similar to investments targeting pollution as well as to share the financial burden for projects with international / basin wide benefits. Additionally, partners from local to basin wide level have to be part of common process of adaptation to climate change.

Hydrology

Hydrological alterations include pressures that are causing changes to the hydrological regime, i.e. quantity and dynamics of water flow and connection to groundwater bodies. Impoundments, water abstractions and hydropeaking were recognized as the main hydrological pressure types:

• Impoundments are caused by barriers that – in addition to interrupting river/habitat continuity – alter the upstream and downstream flow conditions of rivers. The character of the river is changed to lake-like conditions due to decrease of flow velocities and eventual alteration of flow discharge and sedimentation. Additionally, impoundments can also lead to severe changes to the river’s sediment balance and to erosion and deepening processes downstream of the impounded section, inducing a decrease of the water table and consequently, dry out of the adjacent land and wetlands. 

• Water abstractions can significantly reduce the flow and quantity of water. They can also influence on sediment transport and morphological conditions and thus impact the water status in case where the ecological flow of rivers is not guaranteed.

• Hydropeaking is a pressure that occurs in the Danube River Basin, stemming from hydropower generation for the provision of peak electricity supply resulting in artificial water level fluctuation. While hydropeaking is induced locally at the water outflow, hydropeaking waves can migrate far downstream.

River continuity 

Transversal structures in the rivers like dams and weirs are interrupting the longitudinal continuity and therefore could hinder fish from migration. Further effects of transversal structures are also changes in the natural sediment dynamics, reflecting as riverbed incision/aggradation due to the interruption of sediment transport.

Morphology

Morphological alterations include pressures that are causing changes to riverbed, banks and floodplains, i.e. changes of river depth and width variation, structure and substrate of the river bed and structure of the riparian zone. Within morphological alterations also pressures related to disconnection of wetlands/floodplains are included.